Add flag for symbolic color names

This is a rough implementation for issue https://github.com/yihui/tikzDevice/issues/69. I'm waiting for feedback
First question: Well, what would happen now? tikzDevice would (re-)define those anyway. In this case, don't use symbolicColors (it's off per default anyway). I don't worry about that now.

Second: Yes , that sounds good, I'm unsure about the extension, though. Let me implement something that we can talk about.